Most conventional Global Positioning System (GPS) receivers utilize serial correlators in order to acquire, track, and demodulate signals transmitted from GPS satellites. The U.S. Global Positioning System (GPS) Orbital Constellation consists of 24 satellites or space vehicles (SV) which orbit the earth in 12 hour orbits. The satellites are arranged in six orbital planes each containing four satellites. The orbital planes are spaced 60 degrees apart from each other and are inclined approximately fifty-five degrees with respect to the equatorial plane. This constellation provides a user with approximately five to eight satellites visible from any point on earth.
Each transmitted GPS signal is a direct sequence spread spectrum signal. The signal available for commercial use is that associated with Standard Positioning Service (SPS) and utilizes a direct sequence bi-phase spreading signal with a 1.023 Mchip per second spread rate placed upon a carrier at 1575.42 MHz. Each satellite transmits a unique pseudo-random noise code (also referred to as the `Gold` code) which identifies the particular satellite, and allows signals simultaneously transmitted from several satellites to be simultaneously received by a receiver, with little interference from one another. The pseudo-random noise (PN) code sequence length is 1023 chips, corresponding to 1 millisecond time period. In addition, data superimposed on each signal is 50 baud binary phase shift keyed (BPSK) data with bit boundaries aligned with the beginning of a PN frame; 20 PN frames occur over 1 data bit period (20 milliseconds).
A primary goal of a GPS receiver is to determine the time-of-arrival of the PN codes. This is accomplished by comparing (for each received signal) a locally generated PN reference against the received signal and "sliding" the local reference in time until it is time-aligned with the received signal. The two signals are compared with one another by a multiplication and integration process known as the correlation process. When the two signals are time aligned a large output results. Typical serial correlators used in current standard GPS receivers compare the local and received signals one time offset at a given time. If such a comparison is done every half-chip interval, 2046 comparisons (or tests) would be required to completely search over one PN epoch (1 millisecond). Such a search must be done in turn for each of the several of the satellites in view. In addition, errors in received signal frequency often require additional searches to be made for various hypotheses of signal frequency. The time to perform this search may be very lengthy, especially under low input signal-to-noise ratio situations. Conventional GPS receivers utilize a multiplicity of such correlators operating in parallel to speed up the acquisition process. In order to achieve rapid acquisition at very low received signal to noise ratios, an extremely high number (perhaps thousands) of such correlators may be required. A straightforward implementation of such a system would thus result in very complex, expensive circuitry.
It is therefore desirable to simplify the circuitry associated with correlators within GPS receivers. Such an architecture would effectively and efficiently implement the processing functions of a large number of correlators operating in parallel. It is further desirable to integrate a method for tracking received GPS signals, following the acquisition procedure, in which common components, such as common hardware, are used for both the acquisition and tracking functions.